One idea could be a crossover where Goku meets Superman. They could have a friendly sparring match to test each other's strength. Maybe they start off misunderstanding each other's motives but then realize they are both protectors of their respective universes.
One idea could be a crossover where Goku is accidentally transported to the Justice League's universe. He might initially be seen as a threat due to his power but then they realize he's a good - hearted fighter. They could team up to fight a common enemy like Darkseid who has somehow acquired the Dragon Balls and is using them for his evil plans.
One idea could be a crossover where the powerful Saiyans from Dragon Ball Z end up in the Sekirei universe. They might get involved in the Sekirei Plan, either as protectors or accidental disruptors. For example, Goku could befriend some of the Sekirei due to his kind nature and his love for a good fight.
One idea could be that Danny, using his ghost powers, somehow gets transported to the Dragon Ball Z universe. He might initially be mistaken for an otherworldly being. He could team up with Goku and the gang, and use his intangibility and invisibility to help in battles against foes like Frieza or Cell. For example, he could sneak up on enemies and disrupt their energy blasts from within.
Well, in such a fanfiction, the power levels would be a major aspect to consider. Dragon Ball characters often have extremely high power levels that can be measured in numbers, while Justice League characters have different kinds of powers like Batman's gadgets and intelligence. The story could involve a power - level adjustment or some kind of power - sharing device. For example, Vegeta might learn from Batman how to use strategy more effectively in battles, and in return, Batman could gain some ki - based abilities from Vegeta.
One idea could be a crossover where Goku and Naruto meet. Maybe they start off as rivals, each thinking their own world's way of fighting is superior. But then they face a common enemy that forces them to combine their powers. For example, a powerful demon that can absorb chakra and ki. They would have to learn from each other's techniques to defeat it.
One idea could be to have the characters from Dragon Ball Z attend the school in Highschool DxD. For example, Goku could be a transfer student with his incredible power causing quite a stir among the devils, angels and fallen angels. He might be confused by the different social dynamics and the whole concept of the supernatural school setting.
Maybe there's a rift in the multiverse that merges parts of the Dragon Ball Z world and the DC Universe. The Z Fighters and the Justice League initially think they are enemies due to misunderstandings. For instance, when Goku first meets Batman, Batman might mistake Goku's carefree attitude for recklessness. But as they gradually figure out the situation, they team up to close the rift and fight off threats that are emerging from the merged worlds.
A cool plot might involve a rift in space - time that pulls the Dragon Ball Z world and the DC Universe together. Vegeta could be at odds with Wonder Woman at first, seeing her as a rival in terms of strength and combat skills. The Justice League and the Z Fighters then discover that an ancient evil force from both of their universes' pasts has been awakened. They must learn to work together, with Gohan and Robin sharing their intelligence to figure out a plan. The Saiyans' transformation abilities and the Justice League's various powers like the Flash's speed would be crucial in the final battle against this evil.
One idea could be that the Shinju's power somehow connects the Naruto universe and the Justice League's universe. Maybe the Shinju's energy creates a rift, and Naruto characters end up in the Justice League's world. They could team up to fight a common enemy that is attracted by the strange energy of the Shinju. For example, a powerful demon from Naruto's world could be drawn to the new universe and the Justice League, with their high - tech and superpowers, join forces with Naruto and his friends who have ninja skills and chakra abilities.
Another possibility is a post - Endgame fanfiction. After the events of Endgame, the Avengers discover a new threat that has ties to the Justice League's world. Maybe a powerful villain from the DC universe has somehow been awakened by the events in the Marvel universe. So they have to reach out to the Justice League for help and together they figure out a way to stop this new threat.